Multi-layer Membrane And The Use Thereof For The Separation Of Liquid Mixtures According To The Pervaporation Process - Patent 5156740

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Multi-layer Membrane And The Use Thereof For The Separation Of Liquid Mixtures According To The Pervaporation Process - Patent 5156740 Powered By Docstoc
					


United States Patent: 5156740


































 
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	United States Patent 
	5,156,740



 Bruschke
 

 
October 20, 1992




 Multi-layer membrane and the use thereof for the separation of liquid
     mixtures according to the pervaporation process



Abstract

A multi-layer membrane having a porous backing layer of polyacrylonitrile,
     polysulfone or the like, and an active separating layer of polyvinyl
     alcohol or cellulose acetate. The membrane is particularly suitable or the
     separation of water-alcohol mixtures according to the pervaporation
     process.


 
Inventors: 
 Bruschke; Hartmut (Nubloch, DE) 
 Assignee:


GFT Ingenieurburo Fur Industrieanlagenbau
(DE)





Appl. No.:
                    
 07/470,216
  
Filed:
                      
  January 25, 1990

 Related U.S. Patent Documents   
 

Application NumberFiling DatePatent NumberIssue Date
 197055Apr., 1988
 778741Sep., 19854755299
 499847Jun., 1983
 

 
Foreign Application Priority Data   
 

Jun 01, 1982
[DE]
3220570



 



  
Current U.S. Class:
  210/490  ; 210/500.42
  
Current International Class: 
  B01D 69/12&nbsp(20060101); B01D 71/38&nbsp(20060101); B01D 71/16&nbsp(20060101); B01D 61/36&nbsp(20060101); B01D 71/00&nbsp(20060101); B01D 69/00&nbsp(20060101); B01D 061/36&nbsp()
  
Field of Search: 
  
  






 210/640,321.84,490,500.42 427/245,246 264/41
  

References Cited  [Referenced By]
U.S. Patent Documents
 
 
 
3035060
May 1962
Binning et al.

3276996
October 1966
Lazare



 Foreign Patent Documents
 
 
 
0047953
Mar., 1982
EP

2165073
Jul., 1973
DE

2441311
Mar., 1976
DE

2730528
Jan., 1979
DE

2850998
Jun., 1979
DE

23129702
Feb., 1982
DE

52-47579
Apr., 1977
JP



   
 Other References 

S Peter and W. Noetzel, Kolloid-Z. u. Z fur Polymere, 183, p. 97 (1962).
.
S. Peter and H. Fasender, Kolloid-Z u. Z. fur Polymere 196, p. 125 (1963).
.
S. Peter and D. Mittelstadt, Kolloid-Z u. Z. fur Polymere 251, pp. 225-231 (1973).
.
Lobel, et al., "Unstirred-Layer Controlled Transport of Organic Bases and Drugs through Solvent-Type Membranes and the influence of Bound Enzymes," 6 Journal of Membrane Science, pp. 203-233 (1980).
.
H. Eustache, e al., "Separation of Aqueous Organic Mixtures by Pervaportion and Analysis by Mass Spectrometry or a Coupled Gas Chromatograph-Mass Spectrometer," 8 Jr. of Membrane Sc., pp. 105-114 (1981).
.
F. Kainer, Polyvinylalkohole.
.
Dytnerski, Memranprozesse zur Trennung flussiger Gemische, p. 155 (1977).
.
R. Y. M. Huang and N. R. Jarvis, "Separation of Liquid Mixtures Using Polymer Membranes," Journal of Applied Polymer Science, p. 2341 (1970).
.
G. Spencer, Diffusion Coefficients in Water Vapor-Poly(vinyl Alcohol) Systems, Journal of Polymer Science, p. 2067 (1968).
.
T. Sano, Central Research Laboratory Sumitomo Chemical Co., Ltd., "A New Reverse Osmosis Membrane Made of Polyacylnitrile," Chemical Economy & Engineering Review, May 1980..  
  Primary Examiner:  Spear; Frank


  Attorney, Agent or Firm: Foley & Lardner



Parent Case Text



BACKGROUND OF THE INVENTION


This is a divisional application based on U.S. Ser. No. 07/197,055, filed
     Apr. 27, 1988, now abandoned which is a divisional application based on
     U.S. Ser. No. 06/778,741, filed Sep. 23, 1985 (now U.S. Pat. No.
     4,755,299), which is a continuation of Ser. No. 06/499,847, filed Jun. 1,
     1983 (now abandoned).

Claims  

What is claimed is:

1.  A composite membrane consisting essentially of (a) a non-porous separating layer having a thickness between 0.05 and 10 .mu.m, said separating layer being useful for
separating an aqueous liquid mixture comprising at least one organic liquid, said separating layer being comprised of crosslinked polyvinyl alcohol, (b) at least one integral porous backing layer comprised of a second polymer, and (c) at least one
integral porous backing layer comprised of a third polymer, wherein said cross-linked polyvinyl alcohol is disposed on the surface of said porous backing layer (b) so that said separating and backing layers are distinct and wherein said composite
membrane is a flat sheet membrane constructed and arranged for the separation of mixtures in a pervaporation process.


2.  A membrane according to claim 1, wherein said non-porous layer has a selectivity B value of at least 30 for a mixture comprising 80 wt % ethanol and 20 wt % water, relative to the total weight of said mixture.


3.  A membrane according to claim 1, further comprising a carrier layer (d), said carrier layer comprising a fleece or fabric.


4.  A membrane according to claim 3, wherein said third polymer is a polyacrylonitrile.


5.  A membrane according to claim 3, wherein said third polymer is a polysulfone.


6.  A membrane according to claim 3, wherein said carrier layer has a thickness of between 50 and 150 .mu.m and is comprised of a polyester.


7.  A membrane according to claim 1, wherein said third polymer is a polyacrylonitrile.


8.  A membrane according to claim 1, wherein said third polymer is a polysulfone.


9.  A membrane according to claim 1, wherein said porous backing layer (b) has a close distribution of pore radii and has an average pore radius such that polyvinyl alcohol molecules from said non-porous separating layer do not penetrate into the
pores of said porous backing layer (b).


10.  A membrane according to claim 9, wherein said second polymer is a saponified cellulose acetate.


11.  A membrane according to claim 10, wherein said second polymer is saponified cellulose triacetate.


12.  A membrane according to claim 11, said membrane comprising two porous backing layers, at least one of said backing layers being comprised of saponified cellulose triacetate.


13.  A membrane according to claim 1, wherein said cross-linked polyvinyl alcohol is the product of a process comprising (i) a step selected from the group consisting of etherification, esterification with a monobasic or a dibasic acid,
acetalization with a monofunctional or difunctional aldehyde, and a combination thereof, wherein heat is applied during step (i).


14.  A membrane according to claim 1, wherein said cross-linked polyvinyl alcohol is cross-linked with maleic acid.


15.  A membrane according to claim 1, said membrane accommodates a permeate flow of at least 1.5 kg/h.m.sup.2 for said mixture.


16.  A membrane according to claim 1, wherein said non-porous layer has a thickness of between 0.1 and 5 .mu.m.


17.  A membrane according to claim 16, wherein said non-porous separating layer has a thickness between about 1 and 2 .mu.m.


18.  A membrane according to claim 1, wherein said polyvinyl alcohol has a saponification degree in excess of 98 percent.


19.  A membrane according to claim 1 which is produced by dissolving said polyvinyl alcohol in a solvent to form a solution, and applying the solution to backing layer (b).


20.  A membrane according to claim 1, wherein said backing layers have a combined has a thickness of more than 20 .mu.m.  Description  

It is known in the art that liquid mixtures can be separated in
that the liquid mixture is placed into contact with one side of a suitable polymeric membrane while a vacuum is applied to the other side of the polymeric membrane or an inert gas stream is guided past the same.  Some of the components of the liquid
mixture that permeate more rapidly through the polymeric membrane are continuously removed in vaporized condition from the side of the polymeric membrane in communication with the gaseous phase either by evacuation or by the inert gas stream. 
Non-permeating components are retained in the liquid phase accumulating therein until the desired degree of separation of components of a rapid and of a poor permeation and the desired degree of purity of the retained components of an inferior
permeation, respectively, has been attained.


What is particularly noteworthy regarding this process, called liquid permeation or pervaporation and used for the separation of gas mixtures by means of membranes, is the fact that it also permits decomposition of such liquid mixtures into their
components that cannot be separated by simple distillation because they either form azeotropes or the boiling points of the components are so close as to prevent an effective and economical separation.  For, in pervaporation, it is no longer the partial
vapor pressure of the components above the liquid that determines the composition of the mixture in the gaseous phase, but rather the different permeability of the membrane and, hence, the selectivity thereof for the various components in the liquid
mixture.  On laboratory scale, for example, mixtures of benzene/cyclohexane and isopropanol/water could be separated beyond the respective azeotropic mixtures by means of pervaporation.  Similarly, it was possible to separate the xylene isomers o-xylene
(boiling point 144.4.degree.  C.), m-xylene (boiling point 139.1.degree.  C.) and p-xylene (boiling point 138.3.degree.  C.) by pervaporation in laboratory.


A special position is occupied by the mixtures of the simple oxygen-containing organic compounds, e.g. of the simple alcohols, ketones, ethers, aldehydes and acids, with water.  On the one hand, these compounds, frequently, are technically
important substances required in large scale for the most various applications in dry and anhydrous condition; conversely, these compounds, in general, completely or largely are miscible with water forming azeotropic mixtures with water so that the
separation and recovery of the anhydrous organic substances envolve substantial expenditure.  Many attempts have therefore been made to use pervaporation processes for the separation of such mixtures; however, the efforts so far taken have never exceeded
the stage of laboratory tests.  Admittedly, the prior art membranes of cellulose diacetate and triacetate that are also employed otherwise, e.g., in reversing osmosis, for some purposes have an adequate mechanical stability and a satisfactory flow of
permeate; However, the selectivity thereof does not yet permit a large-scale industrial use for pervaporation.  The present invention is now concerned with a multi-layer membrane having a non-porous separating layer from a first polymer and a backing
layer from a second polymer, which is characterized in that the separating layer is comprised of polyvinyl alcohol or cellulose acetate.


SUMMARY OF THE INVENTION


The membrane of the invention is suitable for the separation of liquid mixtures by means of liquid permeation or pervaporation, especially for the separation of water from its mixtures with oxygen-containing, organic liquids, such as simple
alcohols, ethers, ketones, aldehydes or acids.  Moreover, the membrane of the invention is also suitable for the separation of gas mixtures.


The membranes of the invention, owing to their multi-layer structures, on the one hand, have an excellent mechanical stability and, on the other hand, the separating layer can be applied to the mechanically stable backing layer.  With a thickness
which is sufficiently thin to provide selectivity but also to permit industrial use of such membranes.  In technical usage, the multi-layer membranes are also designated as compound or composite membranes.


DETAILED DESCRIPTION OF PREFERRED EMBODIMENT


In accordance with the present invention, the separating layer of the membrane is comprised of polyvinyl alcohol or cellulose acetate.  Polyvinyl alcohol is usefully obtained by saponification of polyvinyl acetate.  Preferably, a polyvinyl
alcohol is used that has a high saponification degree, e.g., a saponification degree in excess of 98 or 99 percent.  The molecular weights are uncritical, if only film formation and membrane formation, respectively, are safeguarded.  Usual molecular
weights are within the range of between 15.000 and 200.000, e.g. between 70.000 and 120.000 (Daltons).  Suitable products are commercially available.  The cellulose acetates, in the first place, are cellulose diacetate and triacetate having the
characteristics normally used in the production of membranes.  Separating layers from polyvinyl alcohol are the preferred ones as they yield substantially improved results both as to selectivity and permeate flow.


In the separating layers it is necessary (as it is with the porous backing layers) that the polymers used in the present invention not be removed or attacked either by water or by the solvents to be separated.


The use of polyvinyl alcohol as a separating layer exhibits quite a number of characteristics.  Polyvinyl alcohol is easily soluble in water for which reason it can in simple manner be applied from an aqueous solution, whereas polyvinyl alcohol
is insoluble in all simple organic solvents.  Polyvinyl alcohol, chemically and thermally, is stable; polyvinyl alcohol layers can be after-treated by cross-linkage to be extent that, in the long run, they are also insoluble in hot water showing only a
swelling with water that can be adjusted by the type of the cross-linking reaction.  Very thin, firmly adhering separating layers of polyvinyl alcohol that have an adequately high permeate flow can be applied to suitable backing layers.  Owing to the
conditions of manufacture, the properties of the polyvinyl alcohol separating layers are widely variable so that the pervaporation membranes produced thereby, with respect to selectivity and permeate flow can be optimally adapted to the respective
problem of separation.


Insolubility of the polyvinyl alcohol in water is caused by cross-linkage.  Preferably, cross-linking is performed by etherification, esterification or acetalization, or by a combined use of the said processes.  Examples are esterification with
dicarboxylic acids, preferably those that, in addition, contain hydroxyl- and/or keto-groups; etherification under catalytic exposure to acids, or by means of dihalogen compounds, such as 1,3-dichloroacetone or 1,3-dichloroisopropanol, or acetalization
by means of aldehydes or dialdehydes.  Based on ethanol/water mixtures, esterification, in general, results in an increased selectivity and acetalization leads to an increase in the rate of permeate flow while the influence of etherification on
selectivity and permeate flow rate is less pronounced.  The afore-going, in analogy, applies to other hydrous mixtures.  The effects of a number of different cross-links are revealed by the examples.


The separating layer of polyvinyl alcohol or cellulose acetate is required to be non-porous and free of defects (free of holes).  The thickness of the separation layer, in general, amounts to between 0.05 and 10 .mu.m, preferably to between 0.1
and 5 .mu.m, with layer thickness of about 1 to 2 .mu.m, in practice, having provided to be particularly suitable.


The non-porous separating layer, dissolved in a suitable solvent, is directly applied to the porous backing layer.  The concentration in this respect is non-critical, however, hardly will it be possible to attain sufficiently thin layers by
excessively viscous solutions.  The preferred solvent for polyvinyl alcohol is water, for cellulose triacetate it is chloroform or trichloroethylene.


Generally, all substances suitable for use as ultrafiltration membranes can be considered as porous backing layers for the multi-layer membranes of the invention.  Owing to the desired thermal stability and the unsensitivity to the solvent
mixtures to be separated, porous backing layers of polyacrylonitrile (PAN), polysulfone (PS) and hydrolyzed or saponified cellulose acetates will be preferred.  Preferably, the porous backing layer has a very close distribution of pore radii with an
average pore radius such that the macromolecules of the polymer used for the separating layer of the membrane, preferably polyvinyl alcohol, cannot penetrate into the pores of the backing layer but are rather retained on the surface.  In this manner,
very uniform, thin and efficient separating layers can be applied to the porous backing layer.  The adjustment of the pore radii and of the distribution of the pore radii, on the one hand, can be effected by corresponding conditions in the preparation of
the porous backing layer as such, or it can be effected by applying an intermediate layer to a less suitable porous backing layer, in which case, the separating layer is applied to the intermediate layer (see e.g. examples 5 and 6 ).  When using
cellulose acetate for the separating layer, pore distribution and average pore radius of the backing layer are less critical, as the molecular weight of cellulose acetate is substantially higher than that of polyvinyl alcohol and, beyond that, also a
different molecular configuration prevails.  Finally, solutions from cellulose acetate in organic solvents are of a higher viscosity than corresponding aqueous polyvinyl alcohol solutions.  For all these reasons, the use of cellulose acetate as a
separating layer is less sensitive regarding the pore distribution and the pore radius of the porous backing layer.


The thickness of the porous backing layer is not critical, if only an adequate mechanical strength of the multi-layer membrane is safeguarded.  The thickness of the porous backing layer amounts to e.g. 20,50 or 100 .mu.m, or more.


In a preferred embodiment, the porous backing layer in the inventive membrane is applied to a fleece or to a fabric serving as a carrying layer.  As are the other layers, the carrier layer, preferably, is resistant to temperature and chemicals. 
The carrier layer, on the layer side, preferably, is smooth to avoid a damage to the porous backing layer.  Polyesters are the preferred ones; cellulose layers, in general, are not sufficiently smooth.  The polyamides actually suitable, in general are
less preferred owing to the thermal resistance thereof lower than that of polyesters and owing to their lower solvent strength.  The thickness of the carrier layer is not critical either; in practice, thicknesses of between 50 and 150 .mu.m, e.g.
approximately 100 .mu.m, have proved to be particularly suitable.


Application and distribution of the polymeric solutions forming the porous backing layer and the non-porous separating layer, generally, are performed in a manner that the polymeric solutions, by way of a knife blade or glass rod, are distributed
over and swept clear of the corresponding layer.  For this, the layer thicknesses are adjusted in accordance with the desired layer thickness.  In addition to that process, in practice, especially for less viscous polymeric solutions, also a process has
proved to be suitable that in the American-language literature has been described as the "meniscus coating" or "dip coating".  The carrier material to be laminated, with the side to be coated is drawn downwardly over a roll just touching the surface of
the polymeric solution to be applied.  A meniscus is then formed between the liquid surface and the carrier material, with the surface of the carrier material being wetted by the polymeric solution.  In some cases, it is preferred to improve the
wettability of the carrier material by the addition of wetting agents.  By varying the viscosity of the polymeric solution, the drawing speed, the carrier material and of the dripping time of the polymeric solution, the thickness of the so applied layer
of the polymeric solution can be widely varied and reproducibly adjusted.


The permeate flow in kg/h.m.sup.2 under the test conditions of the temperature, the composition of the mixture to be separated and of the pressure on the permeate side, and the selectivity B of the membrane under the said conditions serve for
characterizing of pervaporation membranes.  B is a non-dimensional figure representing the concentration ratio of the binary mixture in the permeate divided by the concentration ratio in the feed.  ##EQU1##


The volume of the permeate flow shows heavy dependence on the temperature.  While in all examples of embodiment the feed of the liquid mixture was performed under atmospheric pressure, the pressures prevailing on the permeate side amounted to
between 10 and 50 mbar.  In that range the level of the permeate-sided pressure was without any noteable influence on the permeate flow and the selectivity of the membranes. 

The invention will now be described with reference to the drawings,
wherein


FIG. 1 is a cross-sectional view of a preferred embodiment of a multi-layer membrane of the invention, and


FIG. 2 is a cross-sectional view of another preferred embodiment of a membrane according to the invention. 

According to FIG. 1, a multi-layer membrane 1 is comprised of a polyester fleece carrier layer 2 having a thickness of 120 .mu.m. 
Provided thereon is a porous backing layer 3 of polyacrylonitrile having a thickness of 50 .mu.m.  The separating layer 4 is comprised of polyvinyl alcohol cross-linked with maleic acid and has a thickness of 1,2 .mu.m.  The manufacture of that
multi-layer membrane will be described in example 1.


According to FIG. 2, a multi-layer membrane 21 is comprised of a polyester fleece carrier layer 22 having a thickness of 120 .mu.m.  Provided thereon is a porous backing layer 23 of polyacrylonitrile having a layer thickness of 50 .mu.m. 
Provided thereon is a porous intermediate layer (another backing layer) 24 of saponified cellulose triacetate having a thickness of 50 .mu.m.  A non-porous separating layer 25 of polyvinyl alcohol cross-linked with maleic acid has a layer thickness of
under 1 .mu.m.  The production of that multi-layer membrane will be described in examples 5 and 6.


The examples will explain the invention.


The carrier layer as used in the examples is a polyester fleece having a layer thickness of about 120 .mu.m.


The polyvinyl alcohol (PVA) as used is a commercially available product having a degree of saponification of at least 99 percent and an average molecular weight of 115.000 (Daltons).


EXAMPLE 1


A 15% solution of polyacrylonitrile (PAN) in dimethyl formamide (DMF) by means of a knife blade is applied as a layer of a 50 .mu.m thickness to a polyester fleece forming a carrier layer, and is precipitated according to the phase inversion
process at a temperature of 8.degree.  C. The resulting porous membrane, at a pressure differences of 4 bar, shows a clear water discharge rate of 150 l/h.m.sup.2 and a retention capacity of more than 99.5 percent for a 1% solution PVA in water.


Subsequently applied to that PAN membrane is a solution of 7 percent by weight PVA in water added to which are 0.05 mole maleic acid per mole monomeric unit PVA.  After drying, hardening and cross-linking of the PVA separating layer at
150.degree.  C., the PVA separating layer will no longer be soluble either in boiling water.  Tests with water/alcohol mixtures, at a feed temperature of 80.degree.  C. and a water/alcohol ratio in the feed of 1/4 resulted in a selectivity of B=1400 and
a rate of permeate flow of 0.04 kg/h.m.sup.2.


Under otherwise identical conditions except for a water/alcohol ratio in the feed of 5/95, the selectivity was still 9500 at a rate of permeate flow of 0.01 kg/h.m.sup.2.


EXAMPLE 2


Example 1 is repeated, except for that a reduced PVA concentration of 5% is used in the preparation of the PVA separating layer.  Under the conditions as specified, the following values are measured on the final membrane:


Feed 12 percent by weight water, 88 percent by weight ethanol, feed temperature 80.degree.  C., selectivity 250, rate of permeate flow 0.05 kg/h.m.sup.2.


Feed 20 percent by weight water, 80 percent by weight isopropanol, feed temperature 45.degree.  C., selectivity 250, rate of permeate flow 0.3 kg/h.m.sup.2.


Feed 20 percent by weight water, 80 percent by weight acetone, feed temperature 60.degree.  C., selectivity 100, rate of permeate flow 0.25 kg/h.m.sup.2.


EXAMPLE 3


A PAN membrane produced in accordance with example 1, is coated with an aqueous solution of the following composition: PVA 5 percent by weight; formaldehyde 1 mole per mole PVA monomeric unit; hydrochloric acid 1 mole per mole PVA monomeric unit.


After hardening at 155.degree.  C., the following separating performances are determined for ethanol/water mixtures at a temperature of 70.degree.  C.:


Feed 80 percent by weight ethanol, selectivity 30, rate of permeate flow 1.5 kg/h.m.sup.2.


Feed 90 percent by weight ethanol, selectivity 50, rate of permeate flow 1.0 kg/h.m.sup.2.


Feed 99 percent by weight ethanol, selectivity 30, rate of permeate flow 0.25 kg/h.m.sup.2.


EXAMPLE 4


An 18% solution of polysulfone (see Condensed Chemical Dictionary, 8th edition, 1971, p. 712) in DMF, in accordance with the description in example 1, is applied to polyester fleece as a carrier layer and is precipitated in water at a temperature
of 8.degree.  C. by phase inversion.  Applied to the so formed porous backing layer is an aqueous solution of 6 percent by weight PVA containing 0.05 mole fumaric acid per mole monomeric unit PVA and is hardened at 150.degree.  C. At a temperature of
80.degree.  C. and at a feed concentration of 80 percent by weight ethanol in water, a selectivity of 350 and a permeate flow of 0.2 kg/h.m.sup.2 were measured.


EXAMPLE 5


A 15% solution of PAN in DMF, according to example 1 is applied to a polyester fleece forming the carrier layer and is precipitated in water at a temperature of 15.degree.  C. The so obtained porous membrane shows a clear water discharge of more
than 150 l/h.m.sup.2, a retention capacity for a 1 percent PVA solution of 90 percent at an average molecular weight of the PVA of 11500, and of 50 percent at an average molecular weight of the PVA of 72000.  After drying, a 1% solution of cellulose
triacetate in anhydrous chloroform is applied to the said membrane by "dip-coating", and the solvent is evaporated under the exclusion of moisture.  At a temperature of 80.degree.  C., the said membrance, at a feed concentration of 80 percent ethanol in
water, showed a selectivity of 10 at a permeate rate of flow of 2 kg/h.m.sup.2.


EXAMPLE 6


The membrane provided with a separating layer of cellulose triacetate obtained according to example 5 was exposed to an aqueous ammonia solution of a pH-value of 10.5 until complete saponification of the cellulose triacetate.  Now the said porous
membrane, for PVA of a molecular weight of 115000, showed a retention capacity of in excess of 99.5 percent and, for PVA of a molecular weight of 72000, showed a retention capacity of 98 percent.  Coated with a 3 percent PVA solution containing maleic
acid as the cross-linking agent, at a feed temperature of 78.degree.  C. and at an ethanol concentration of 80 percent, a selectivity of 250 was obtained at a permeate flow rate of 0.5 kg/h.m.sup.2.


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DOCUMENT INFO
Description: It is known in the art that liquid mixtures can be separated inthat the liquid mixture is placed into contact with one side of a suitable polymeric membrane while a vacuum is applied to the other side of the polymeric membrane or an inert gas stream is guided past the same. Some of the components of the liquidmixture that permeate more rapidly through the polymeric membrane are continuously removed in vaporized condition from the side of the polymeric membrane in communication with the gaseous phase either by evacuation or by the inert gas stream. Non-permeating components are retained in the liquid phase accumulating therein until the desired degree of separation of components of a rapid and of a poor permeation and the desired degree of purity of the retained components of an inferiorpermeation, respectively, has been attained.What is particularly noteworthy regarding this process, called liquid permeation or pervaporation and used for the separation of gas mixtures by means of membranes, is the fact that it also permits decomposition of such liquid mixtures into theircomponents that cannot be separated by simple distillation because they either form azeotropes or the boiling points of the components are so close as to prevent an effective and economical separation. For, in pervaporation, it is no longer the partialvapor pressure of the components above the liquid that determines the composition of the mixture in the gaseous phase, but rather the different permeability of the membrane and, hence, the selectivity thereof for the various components in the liquidmixture. On laboratory scale, for example, mixtures of benzene/cyclohexane and isopropanol/water could be separated beyond the respective azeotropic mixtures by means of pervaporation. Similarly, it was possible to separate the xylene isomers o-xylene(boiling point 144.4.degree. C.), m-xylene (boiling point 139.1.degree. C.) and p-xylene (boiling point 138.3.degree. C.) by pervaporation in laboratory.A